Rotating electrical machines are widely known in the art, and basically include a static part (stator) and a rotating part (rotor). In a simplified version, the stator is made up of a slotted magnetic core where coils or windings are placed, and the rotor consists of a slotted magnetic core that can be filled with coils or windings, or filled with liquid metal and then so sealed. This assembly is mounted on an axle, thus producing the rotor.
Explosive atmospheres are those that contain, in addition to oxygen, a proportion of gas, steam, dust or fibers, where a spark from an electrical circuit or the heating of a device can be an ignition source or cause an explosion. This kind of atmosphere is found, for example, in petrochemical, food, pharmaceutical, paper, and textile industries.
A “combustion process” depends upon the rate of flame propagation and to the generated pressures. Usually, combustion processes with flame propagation speeds of up to 2 m/s and pressures below 4 bar are referred to as “burning”. For speeds between 0.5 and 4.0 m/s at pressures ranging between 4.0 and 10.0 bar the term “explosion” is used. The term “blasting” is adopted when the pressure exceeds 10.0 bar and speeds exceed 30 m/s and even exceed the supersonic speed. As used herein, the term “explosion” encompasses any type of combustion process.
The electrical machines designed to operate in this kind of atmosphere are built to avoid or reduce the risk of explosion. One known way to achieve this is to confine the parts that can cause the ignition of an explosive atmosphere within a casing capable of withstanding the pressure generated by an internal explosion without allowing it to spread to the external environment. The machine casing is capable of withstanding the pressure wave generated by the explosion inside. Usually, the thickness of the casing is increased to make it able to withstand the mechanical stresses from this explosion process. Thus, the intention is to limit the effect—in this case, the pressure generated by the combustion process, and not the cause—the increase in pressure.
Increasing the thickness of the casing causes an increase in weight and volume of the rotating electrical machine, and consequently, operational restrictions and costs increase. In some cases, performing prior tests is required to ensure that the components of the casing are able to withstand the occurrence of explosion. These tests, which can be performed on the electrical machine components and/or on the electrical machine after assembled, result in a higher manufacturing costs and higher manufacturing times, thus directly impacting the competitiveness of the product.
European document EP 0113628 discloses an electrical rotary machine with an explosion-proof casing formed with shields. The shields include a surface substantially perpendicular to the axis of the rotor, and an outer cylindrical surface, and an inner cylindrical surface, each of these cylindrical surfaces being parallel to the axis of the rotor. The explosion-proof casing is constituted by the casing and fixed shields, the cylindrical surfaces being at the same side of their plane surface and the outer cylindrical surface being situated in the immediate vicinity of the casing.
These shields may perhaps be a more economical solution than the one involving a casing with a greater thickness, but use of the reinforcement shields still causes a significant increase of the whole weight of the machine.